Upward increase of kerogen delta C-13 in the Peru Margin Upper Oligocene: possible implications for the Cenozoic evolution of atmospheric CO2

Carbon isotopic composition of predominantly marine kerogen in latest Oligo cene mudstones of the Peru Margin ODP 682A Hole shows an about 3.5 parts pe r thousand increase with decreasing age. Py-GC and elemental (C/N ratio) an alysis of the kerogen plus sulphur isotopic study together with earlier kno wledge on geological setting and organic geochemistry results in a better u nderstanding of depositionary environment and allows to separation of the i nfluence of concentration of water dissolved carbon dioxide (c(e)) on kerog en delta(13)C from that of other factors (bacterial degradation, sea surfac e temperature, DIC delta(13)C, productivity, and admixture of land plant OM ). Based on this analysis, the major part of the kerogen shift is considere d as a result of the latest Oligocene decrease of marine photosynthetic car bon isotopic fractionation in the Peru Margin photic zone, which in turn po ssibly reflects a simultaneous drop in atmospheric CO2 level. Uncertainties in the evaluation of the factors affecting the marine photosynthetic carbo n isotopic fractionation and the extent of ocean-atmosphere disequilibrium do not permit calculation of the decrease of the atmospheric CO2. (C) 1999 Elsevier Science B.V. All rights reserved.